A typical pilot operated jerk steer system for a mobile machine includes a pilot operated steering valve for controlling flow of fluid from a steering pump to the steering cylinders and from the steering cylinders to a tank. The position of the steering control stem of the steering valve is controlled by a pilot valve which sends pressurized oil to one end of the control stem for shifting the control stem. Steering is typically controlled by manual manipulation of a steering wheel connected to the pilot valve through suitable mechanical linkage or in some instances, by manual manipulation of a steering lever mechanically connected to the pilot valve.
Steering to the right, for example, is initiated by rotating the steering wheel rightward so that the pilot valve sends pressurized pilot oil through a right steer pilot line to the right steer end of the control stem. However, the left steer end of the control stem has to be vented to the tank in order for the control stem to move in the right steer direction. In the early steering systems, the left steer end was typically vented to tank through the pilot valve. This created a problem in that the oil was vented so quickly that the control stem traveled further than intended (overshoot) as it shifted in the right steer direction. The control stem overshoot caused more oil to flow from the pump to the steering cylinders than intended when a right turn was initiated resulting in jerky or unpredictable steering. The rate at which the vehicle accelerated when steering begins is often referred to as "start steer jerk". The same reaction occurs when a left steer operation is initiated.
In a jerk steer system, the right steer motion is stopped by returning the pilot valve to its neutral position to stop supplying pressurized pilot oil to the control stem and to connect both ends of the control stem to the tank through the pilot valve. The centering spring quickly returns the control stem to its neutral position to block pump-to-cylinder and cylinder-to-tank flow. This created another problem in that quickly returning the control stem to its neutral position caused rapid deceleration of the steering motion. The rate at which the steering decelerates when the operator stops the steering motion is often referred to as "stop steer jerk".
Both the start steer jerk and stop steer jerk can be alleviated somewhat by adding check valves in the pilot lines between the pilot valve and the ends of the control stem and adding a dampening orifice between the tank and each of the pilot lines. The check valves block the flow of oil from the ends of the control stem to the pilot valve thereby, forcing the oil exhausted from the ends of the control stem to pass through the dampening orifice. For example, the left steer dampening orifice controls the amount of overshoot and thus the start steer jerk when a right steer operation is initiated, the right steer dampening orifice controls the rate at which the steer valve returns to the neutral position from a right steer direction and thus controls the stop steer jerk, the right steer dampening orifice controls the amount of overshoot and thus the start steer jerk when a left steer operation is initiated and the left steer dampening orifice controls the rate at which the control stem returns to the neutral position from a left steer direction and thus the stop steer jerk.
One of the problems with the steering systems having the check valves in the pilot lines and the dampening orifices is that the start steer jerk levels and the stop steer jerk levels cannot be controlled independent from one another because they are both controlled by a fixed area dampening orifice on each side of the control stem. The orifice diameters must be equal to provide equal dampening for left and right steering. A very small orifice diameter on both orifices will provide a very low stop steer jerk which is desirable but will also reduce the start steer jerk levels and the start steer response to unacceptable low levels. Fast start steer response which results in some amount of start steer jerk is required to provided adequate control of the machine. However, a dampening orifice is still required to control the start steer jerk and start steer response so that the steering system is not over-responsive resulting in jerky, uncontrollable steering. A large dampening orifice diameter that provides acceptable start steer jerk and start steer response results in unacceptable high stop steer jerk levels because the large dampening orifice would allow the control stem to return to the neutral position too quickly causing a high rate of steering deceleration. The typical pilot operated jerk steering system must use a dampening orifice diameter that provides compromised performance of star steer jerk, start steer response and stop steer jerk since no one parameter can be set to optimum performance without sacrificing performance of the other parameter.
The present invention is direction to overcoming one or more of the problems as set forth above.